The formation of tabular silver halide photographic emulsions generally comprises of three main steps. These steps are, as described in U.S. Pat. No. 4,797,354 (Saitou, Urabe and Ozeki, 1989) (a) the nucleation step whereby the conditions are selected to generate mostly doubly twinned nuclei with parallel twin planes, which are suitable for producing tabular grains; (b) the ripening step whereby the conditions are changed to promote the dissolution of any nuclei that are not suitable for forming tabular grains (e.g., multiply twinned nuclei with nonparallel twin planes, singly twinned nuclei, octahedral and cuboctahedral nuclei), so that a high population of tabular crystals is achieved; and (c) the growth step whereby the surviving tabular grain nuclei are grown in size without changing their total number, by adding silver and halide reactants at rates which do not exceed the maximum growth rate as described by Wey and Strong in "Growth Mechanism of AgBr Crystals in Gelatin Solution", Photographic Science and Engineering, Vol. 21, 1977, pp. 14-18.
The nucleation and ripening steps are very important because they determine the final stable number of tabular crystals, and hence the average grain volume per mass of silver reactant added, as well as the tabular grain population of the final emulsion. Of these two steps, the nucleation step is the most critical because the effect of the ripening step is limited to reducing nontabular grain nuclei.
The final tabular grain population of AgBr emulsions containing small amounts of iodide and/or chloride is consequently, largely dependent on the nucleation step, which can be carried out by the single-jet method, where silver reactant is added to a well-mixed solution of gelatin, or other appropriate peptizer, and halide, or by the double-jet method, where silver and halide reactants are simultaneously added to a well-mixed solution of gelatin, or other appropriate peptizer, at a controlled pBr, as described by Duffin in "Photographic Emulsion Chemistry", Ch. IV, 1966, by Berry in "The Theory of the Photographic Process", Ch. 3, T. H. James (Ed.), 4th Ed. 1977, and by Wey in "Preparation and Properties of Solid State Materials", Vol. 6, Ch. 2, W. R. Wilcox (Ed.), 1981. The nucleation step can also be carried out by a dual-zone process, using a two-reactor system, where the silver reactant, the halide reactant, and gelatin or other appropriate peptizer are first mixed in a continuous reactor and then added to a second semi-batch reactor, which is used in the nucleation step as a holding vessel, and subsequently as a growth vessel, as described in U.S. Pat. No. 5,035,991 (Ichikawa, Ohnishi, Urabe, Kojima and Katoh, 1991) and U.S. Pat. No. 5,104,785 (Ichikawa, Ohnishi, Urabe and Katoh, 1992).
It is well known that there are several factors during the nucleation step that facilitate the formation of a large population of twinned AgBr nuclei which are suitable for growth to tabular crystals. Several of these factors which are important in the double-jet nucleation method are given in U.S. Pat. No. 4,945,037 (Saitou, 1990) col. 12, line 46, to col. 13, line 40, and of these the most important are the gelatin concentration, the rate of agitation in the nucleation vessel, the silver reactant addition rate, the temperature, the pBr, the presence of halides other than bromide, the pH, and the gelatin type.